Conventional captioning systems allow a separate text channel to be sent (or encoded) within a video signal. EIA-608, also known as “line 21 captions” and “CEA-608”, are standards for closed captioning for NTSC TV broadcasts in the U.S., Canada, and Mexico. EIA-608 defines four possible caption “channels”, although many televisions only conveniently offer two (captions enabled, captions disabled, alternate caption channel enabled—typically Spanish). CEA-708 is the standard for closed captioning for ATSC digital television (DTV) streams in the U.S. and Canada. There are quite a few video and internet-connected devices that, for various reasons, only offer a subset of the full captioning/subtitling feature set available under the standards. CEA-708 defines 63 possible channels, but many televisions only offer enabled/disabled at the convenient touch of a button, with the specific page selection hidden away on an obscure menu. Apple devices (i.e., iPhone, iPad) will play back 608 captions, but only the first channel. Many other cable boxes and software players behave similarly to televisions that make basic enable/disable selections convenient, but channel selection of a particular caption difficult or not even possible.
In conventional approaches, 708 captions (as well as 708-wrapped-608) are attached to the elementary video stream by the video encoder. The space the captions use is accounted for in the overall bandwidth of the stream. Typically, the encoder passes through captions from the input video, or merges the captions from a secondary source (i.e., SCC files). The caption bytes get attached to the video. The captions are not modified again. The device playing the stream can then ignore the captions or select an individual caption channel for playback. In conventional systems, selecting caption channels beyond the first channel is either difficult or not possible, depending on the feature set implemented in the end device.
It would be desirable to implement a captioning system that allows a source device to place a selectable caption channel in a priority position in a video stream.